The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosures of all references cited herein are incorporated by reference.
Chlorination of public water supplies has been practiced for almost 100 years in the United States. In that regard, chlorine is an oxidant that kills many harmful microorganisms. Although the pros and cons of disinfection with chlorine have been extensively debated, it remains the most widely used chemical for disinfection of water in the U.S.
Chlorine usually is added to water as the gaseous form or as sodium or calcium hypochlorite. Chlorine gas rapidly hydrolyzes to hypochlorous acid according to the following equation:Cl2+H2O→HOCl+H++Cl−
Similarly, aqueous solutions of sodium or calcium hypochlorite will hydrolyze according to:Ca(OCl)2+2H2O→Ca2++2HOCl+2OH−NaOCl+H2O→Na++HOCl+OH−
The two chemical species formed by chlorine in water, hypochlorous acid (HOCl) and hypochlorite ion (OCl−), are commonly referred to as “free available” chlorine. Hypochlorous acid is a weak acid and will disassociate according to:HOClH++OCl−In waters with pH between 6.5 and 8.5, the reaction is incomplete and both species (HOCl and OCl−) will be present. Hypochlorous acid is the more germicidal of the two.
A relatively strong oxidizing agent, chlorine can react with a wide variety of compounds. Of particular importance in disinfection is the chlorine reaction with nitrogenous compounds, such as ammonia, nitrites and amino acids. Ammonia, commonly present in natural waters, will react with hypochlorous acid or hypochlorite ion to form monochloramine, dichloramine and trichloramine, depending on several factors such as pH and temperature. In breakpoint chlorination, a continual addition of chlorine to the water up to the point where the chlorine requirement is met and all present ammonia is oxidized, so that only free chlorine remains. After the break point, free chlorine (hypochlorous acid plus hypochlorite) is the dominant disinfectant. The free chlorine residual may, for example, be adjusted to maintain a minimum level of 0.2 mg/L Cl2 throughout the distribution system. The importance of break-point chlorination lies in the control of taste and odor and increased germicidal efficiency. The killing power of chlorine in excess of the breakpoint is 25 times higher than that before the breakpoint is reached. Thus, the presence of a free chlorine residual is an indicator of adequate disinfection.
The use of monochloramine as an alternate disinfectant for drinking water has received attention lately as a result of concern about the possible formation of chlorinated by-products when using free chlorine disinfection. However, considerable debate continues regarding the merits of chloramination disinfection.
A standard for free and total chlorine measurement in water is DPD (N,N-diethyl-p-phenylenediamine) colorimetric detection. Total chlorine is the total amount of chlorine in the water including the chlorine that has reacted with nitrogen compounds in the water. In the absence of iodide ion, free chlorine reacts quickly with DPD indicator to produce a red color, whereas chloramines react more slowly. If a small amount of iodide ion is added, chloramines also react to produce color, yielding total chlorine concentration. Absorbance (for example, at 515 nm) may be spectrophotometrically measured and compared to a series of standards, using a graph or a regression analysis calculation to determine free and/or total chlorine concentration.
As set forth above, free chlorine reacts very quickly with DPD while the chloramine species (for example, monochloramine and dichloramine) react more slowly. In attempting to measure free chlorine, the presence of “interfering” species such as monochloramine may produce inaccurate readings. For greatest accuracy, it is typically recommended that the free chlorine measurement using DPD should be made quickly (that is, before the interfering species can react to any significant degree).
Alternatively, additional reagents may be added to prevent reaction of interfering species, pre-determine the amounts of interfering species, or post-determine the amounts of interfering species. For example, sodium arsenite or thioacetamide can be used to reduce the effects of or prevent interference by monochloramine. However, such additional reagents can be toxic and/or expensive.
Current methods, systems and kits for free chlorine measurement using the DPD colorimetric test are limited because the presence of chloramines can introduce significant errors in free chlorine measurements. Once again, if additional reagents are used to prevent interferences, then additional steps and/or toxic and expensive chemicals are required. Further, the traditional DPD colorimetric test does not allow monochloramine concentration to be measured directly.